Computer disks
If an item is accompanied by two different items in the same format, differentiate them either with terms or numbering from the pieces themselves, or arbitrarily with whatever makes the most sense. Example: a book accompanied by two computer disks, neither one numbered: use "disk 1" and "disk 2."
If it is clear that two or more accompanying computer disks have the same contents but use different platforms (e.g. IBM, MAC, Windows), label them so that the distinction is clear, assigning numbers to the disks only when necessary:
IBM disk, MAC disk, Windows disk.
IBM disk 1, IBM disk 2, MAC disk 1, MAC disk 2